1. Field of the Invention
The present invention relates to a detensioning apparatus and method for the repositioning of a clamp on a prestressed strand.
2. The Prior Art
The use of prestressed concrete is a common and vital construction technique with which high strength structures are formed. Prestressed concrete is an often preferred alternative to reinforced concrete for its increased strength and improved durability with respect to exposure to natural elements, such as moisture, freezing, thawing, and corrosion, and in particular is more resilient to cracking.
Formation of prestressed concrete structures involves the use of a form or xe2x80x9ccasting bedxe2x80x9d. As shown in FIG. 13, the casting bed may be categorized as either a self-stressing prestress casting bed (Type 1xe2x80x94Diagram A) or an abutment anchorage prestress casting bed (Type 2xe2x80x94Diagram B). Please note that any dimensions or numerical values shown in FIG. 13 are given by way of example, and are not intended to limit the invention, as described elsewhere herein.
The Type 1 self-stressing prestress casting bed of Diagram A is formed typically from a steel form, that is provided with heavy steel plates positioned at its ends. One plate is the xe2x80x9cdeadxe2x80x9d end and the other is the xe2x80x9clivexe2x80x9d end, at which manipulation of the prestressing wires is accomplished. The Type 2 abutment anchorage prestress casting bed employs abutments embedded in the ground, to form the ends of the casting form. Again, one abutment acts as the dead end and the opposite abutment acts as the live end.
Steel strands (each comprising one or more wires) are passed through the steel plates or abutments, to extend through the form where the concrete is to be poured. The steel strands are typically tensioned for times ranging from 2 to 12+ hours, prior to pouring the concrete. The strands are secured in a stressed position by placing wedge-shaped steel jaw(s) around the circumference of each strand. The jaws forms a chuck that engages a conical bore of a strand chuck barrel, which is secured against the live end of the casting bed, in such a manner that the pulling force of the strand on the chuck is translated into compressive pressure on the strand to bind the chuck thereto. Tension may be applied by placing a second, xe2x80x9ctemporaryxe2x80x9d chuck on each strand, to the outside of the respective xe2x80x9cpermanentxe2x80x9d chucks. Tension may be applied by a hydraulic ram, pushing against the temporary chuck, relative to the live end, to stretch the strand. The xe2x80x9cpermanentxe2x80x9d chuck is then placed against or extremely close to the outer surface of the live end. The tension exerted by the hydraulic ram is released, enabling the strand to contract a small amount, sufficient to cause the jaws in the xe2x80x9cpermanentxe2x80x9d chuck to engage the strand and be pulled tightly against the strand chuck barrel. The amount by which each strand is allowed to recover is less than the amount by which each strand was originally stretched, so that tension remains, pulling the xe2x80x9cpermanentxe2x80x9d chucks against the live end.
After concrete is poured into the pre-cast concrete form structure, the concrete is allowed to cure. The xe2x80x9cpermanentxe2x80x9d chucks are then removed from the strands.
On certain occasions, the stressed strands must be moved for realignment, or the amount of tension must be reduced, before the concrete is placed in the casting bed. Typically, the procedure of detensioning a stressed strand involves the practice of over-tensioning the strand; thereby stretching it and removing pressure from the steel chuck. In so doing, as a result of the high compressive pressures applied thereto, the chuck often binds to the strand chuck barrel""s conical inner surface and subsequently to the prestressed strand. In the prior art chuck removal involves manual manipulation of the chuck before tension on the strand is released.
This method of detensioning a stressed strand is exceptionally dangerous. During the time at which the chucks must be removed, the strand is under extreme tension. Consequently, any failure of the strand while a worker is manually removing the chuck could potentially result in severe injury or death to a worker, or other damage. Furthermore, when the one or more jaws forming the chuck bind to the strand, the substantial manual manipulation by a worker required breaking the chuck free further increases the danger. Accordingly, significant danger, and effort became associated with the detensioning procedure.
U.S. Pat. No. 3,912,496 discloses a gripping device in which a pull tube extends outwardly from the device and allows for the engagement of a prying tool. The force exerted by a worker on the prying tool directly results in the axial movement of the chuck, thereby releasing the strand from the chuck""s grip. However, this invention requires the positioning of the worker in close proximity to the strand while it is under extreme tension, thereby increasing the potential for serious bodily harm if strand failure were to occur while the worker manually pried the chuck out from the strand chuck barrel.
U.S. Pat. No. 3,478,396 teaches a gripping device that incorporates an externally threaded strand chuck barrel, in which a chuck fit into the conical bore, and a matching internally threaded collar. The threaded housing and collar allows for the re-tensioning of the strand by a worker manually rotating the threaded collar with respect to the threaded bushing.
U.S. Pat. No 5,594,977 describes the combination of engagement of threaded members that, upon rotation, forces a pull tube to axially withdraw the chuck from the conical bore thereby resulting in the release of the strand from the chuck""s grip. These detensioning devices however are permanently integrated into the strand chuck housing, resulting in not only more complex and expensive devices, but also potential corrosion of the threaded members, thereby potentially rendering the invention difficult to use or requiring additional maintenance. Furthermore, the worker must manually manipulate these devices while the strand is under full load, thereby increasing the risk of potential physical harm to the worker from strand failure.
U.S. Pat. No. 3,910,546, and U.S. Pat. No. 3,965,542, both disclose a gripper device incorporating a chuck that can be physically pulled back by a pull rod, which engages channels in the rear edge of the chuck. Neither of these devices can be used in conjunction with a member that passes completely through the channel formed by the chuck, as would be the case with a tensioning strand in actual prestressed concrete construction.
U.S. Pat. No. 3,778,869 teaches the use of a detensioning screw that can be turned to advance or withdraw a pushplate against cylindrical lugs bearing outwardly against the chuck through which the strands pass. These lugs in turn force the chuck rearward from the conical bore, and thereby release the strand from them. However the detensioning aspect is permanently integrated into this device and therefore may not be removed for the purpose of detensioning other chucks.
U.S. Pat. No. 2,245,316 discloses an apparatus that facilitates the removal of a clamping chuck in tensile testing machinery. The angles of the conical bore of the strand chuck barrel are significantly larger than that of the chuck""s outer surface so that when the force on a strand passing through the chuck is released, the radial forces exerted by the chuck forces the body with the greater taper, the strand chuck barrel, to be pushed away, whereby the strand is released from the chuck. However, this device does not consider the situation where an extended period of compressive pressure has bound the chuck to the inner conical surface of the strand chuck barrel and the prestressed strand.
U.S. Pat. No. 4,114,242 discloses an apparatus requiring physical interlocking between the chuck and an external tube. The apparatus involves a tube having an initial flare that engages with a groove on the inside surface of the chuck, and a second flare engaging a mounting plate, which, after the first flare is engaged with the chuck, is pulled rearward to remove the chuck from the bore. This device appears to require a large and substantial apparatus that achieves chuck removal by physical engagement.
Similar to 4,114,242, U.S. Pat. No. 4,454,633 incorporates a latching mechanism for engaging, and subsequently tipping the chuck along their rear surface, thereby releasing an array of prestressed strands from the chuck""s gripping surface. Again, this device requires the physical engagement of the chuck and appears to require a comparatively large and complex mechanism.
It would be desirable to provide a simple, inexpensive, re-useable, and efficient apparatus that allows for the detensioning of prestressed strand without the potential hazards associated with manual chuck manipulation.
It would further be desirable to provide a reliable means for inducing chuck movement, and subsequent release of a prestressed strand from the chuck""s compressive force, without direct physical engagement or manual manipulation of said chuck.
It would also be desirable to provide a simple compact design to allow for the engagement of the apparatus in a grid of strand chuck housings without interference from neighboring strand chuck barrels.
These and other desirable characteristics of the present invention will become apparent in view of the present specification, including the claims, and drawings.
The proposed invention is directed to an improved apparatus for detensioning a stressed member and facilitating subsequent repositioning or removal of the chuck from the strand, wherein a worker need not directly handle or manipulate the chuck during the chuck removal or repositioning process. As in the prior art, a strand to be de-tensioned is first over-tensioned, thereby releasing the pressure from the chuck. However, with the proposed invention, an attraction member within the detensioning sleeve induces separation of the jaws of the chuck from the strand. Removal of the chuck is quick, easy, and does not require dangerous manual manipulation near the over-tensioned strand.
The present invention is directed, in part, to a detensioning apparatus, for use during a process of fabricating prestressed concrete, during which process a region in which concrete is to be poured includes at least two opposing abutments, one or more prestressing strands are run between opposing abutments, tensioned and held in tension, by clamps that engage the strands and bear against the opposing abutments, while concrete is poured into the region and cured, wherein each clamp includes a chuck having one or more jaws, and a surrounding strand chuck barrel, configured so that tension in the strand, between opposing abutments, prompts the chuck to be pressed against the strand to frictionally engage the strand.
The detensioning apparatus preferably comprises a collar member, operably configured to be placed onto a prestressing strand, that has been tensioned and to which a strand clamp has been affixed. The collar member has a central bore, a first portion of which is operably configured to insertingly receive a strand chuck barrel, and a second portion of which is provided with a smaller diameter than the first portion, defining a shoulder configured to bear against at least a portion of an end of a strand chuck barrel.
A separation member housing is operably configured to be insertingly received in the second portion of the central bore of the collar member. At least one separation member is supported on the separation member housing, and operably configured for separating a chuck from a prestressing strand, after the tension in the strand causing the chuck to be prompted against the strand is balanced.
The collar member is operably configured to be slid onto a free end of a prestressing strand, to a position surrounding a strand chuck barrel that is holding a strand in tension, such that upon exertion of tension between a free end of the strand and a strand chuck barrel, the collar member bears against the strand chuck barrel and prevents its movement, so that additional tension in the strand will cause the chuck to be drawn toward the at least one separation member contained within the collar member.
When the chuck of the strand clamp is fabricated at least in part, from magnetically responsive material, the at least one separation member preferably comprises at least one magnetic member mounted on the separation member housing, so as to exert magnetic force on the one or more jaws, when the additional tension is applied to the strand the one or more jaws move with the strand until they come into contact with the at least one separation member, the at least one separation member holding the one or more jaws, allowing the strand to slide through the one or more jaws, thereby detensioning the strand.
In a preferred embodiment of the invention, the separation member housing comprises a first cylindrical member, having a central bore, for receiving a prestressing strand. The first cylindrical member has a diameter less than the diameter of the second portion of the central bore of the collar member, to enable insertion of the first cylindrical member into the second portion of the central bore of the collar member. The first cylindrical member further has a first end operably configured to support the at least one separation member, at a position proximate an outer end of the first portion of the central bore of the collar member. The first end of the first cylindrical member may be provided with at least one cavity operably configured to receive the at least one separation member.
The separation member housing preferably comprises a second cylindrical member, having a central bore operably configured to receive the first cylindrical member, the second cylindrical member having an outer diameter operably configured to enable, at least a first end portion of the second cylindrical member to be insertingly received, at least in part, in the second portion of the central bore of the collar member.
The separation member housing further preferably comprises a first attachment flange, operably connected to a second end of the second cylindrical member, and having an outer diameter greater than the diameter of the second portion of the central bore of the collar member, the attachment flange further having a central bore for enabling the passage therethrough of a prestressing strand.
The detensioning apparatus further preferably comprises a second attachment flange, operably mounted on and having an outer diameter greater than, the collar member, for enabling abutment of the first attachment flange to the second attachment flange, upon insertion of the separation member housing into the second portion of the central bore of the collar member, whereupon affixation of the second attachment flange to the first attachment flange affixes the separation member housing to the collar member.
In a preferred embodiment of the invention, at least a portion of an outer surface of the collar member may be provided with flattened sides, for facilitating placement of the detensioning device in position over a strand chuck barrel and against an abutment.
The present invention is also directed to a method for detensioning a prestressing strand, for use during a process of fabricating prestressed concrete, during which process a region in which concrete is to be poured includes at least two opposing abutments, one or more prestressing strands are run between opposing abutments, tensioned and held in tension, by clamps that engage the strands and bear against the opposing abutments, while concrete is poured into the region and cured, wherein each clamp includes a chuck having from one or more jaws and a surrounding strand chuck barrel, configured so that tension in the strand, between opposing abutments, prompts the chuck to be pressed against the strand to frictionally engage the strand.
The method comprises the steps of:
positioning a detensioning apparatus onto a prestressing strand, which strand has been tensioned and to which a strand clamp has been affixed,
the detensioning apparatus including a collar member, operably configured to be placed onto a prestressing strand, that has been tensioned and to which a strand clamp has been affixed, the collar member having a central bore, a first portion of which is operably configured to insertingly receive a strand chuck barrel, and a second portion of which is provided with a smaller diameter than the first portion, defining a shoulder configured to bear against at least a portion of an end of a strand chuck barrel, a separation member housing, operably configured to be insertingly received in the second portion of the central bore of the collar member, at least one separation member supported on the separation member housing, and operably configured for separating a chuck from a prestressing strand, after the tension in the strand causing the chuck to be prompted against the strand is balanced, the collar member being operably configured to be slid onto a free end of a prestressing strand, to a position surrounding a strand chuck barrel that is holding a strand in tension, such that upon exertion of tension between a free end of the strand and a strand chuck barrel, the collar member bears against the strand chuck barrel and prevents its movement, so that additional tension in the strand will cause the chuck to be drawn toward the at least one separation member contained within the collar member;
sliding the collar member onto a free end of a prestressing strand, to a position surrounding a strand chuck barrel, and affixing the detensioning apparatus against an abutment;
pulling on a free end of the prestressing strand, until the tension in the strand that causes the chuck to be prompted against the prestressing strand has been balanced,
applying additional tension in the strand, to cause the chuck to be drawn toward the at least one separation member contained within the collar member;
whereupon the at least one separation member causes the chuck to become separated from the strand, to enable repositioning of the prestressing strand and/or the strand clamp.
When the chuck of the strand clamp is fabricated at least in part from magnetically responsive material, the method further preferably comprises the step of fabricating the at least one separation member as at least one magnetic member mounted on the separation member housing, so as to exert magnetic force on the one or more jaws, when the additional tension is applied to the strand the one or more jaws move with the strand until they come into contact with the at least one separation member, the at least one separation member holding the one or more jaws, allowing the strand to slide through the one or more jaws, thereby detensioning the strand.
The method further preferably comprises the step of fabricating the separation member housing as a first cylindrical member, having a central bore, for receiving a prestressing strand, the first cylindrical member having a diameter less than the diameter of the second portion of the central bore of the collar member, to enable insertion of the first cylindrical member into the second portion of the central bore of the collar member, the first cylindrical member further having a first end operably configured to support the at least one separation member, at a position proximate an outer end of the first portion of the central bore of the collar member.
The method further preferably comprises the step of providing the first end of the first cylindrical member with at least one cavity operably configured to receive the at least one separation member.
The method further preferably comprises the step of providing the separation member housing with a second cylindrical member, having a central bore operably configured to receive the first cylindrical member, the second cylindrical member having an outer diameter operably configured to enable at least a first end portion of the second cylindrical member to be insertingly received, at least in part, in the second portion of the central bore of the collar member.
The method further preferably comprises the step of operably connecting a first attachment flange, to a second end of the second cylindrical member, and having an outer diameter greater than the diameter of the second portion of the central bore of the collar member, the attachment flange further having a central bore for enabling the passage therethrough of a prestressing strand.
The method further preferably comprises the step of operably mounting a second attachment flange on the collar member, the second attachment flange having an outer diameter greater than the collar member, for enabling abutment of the first attachment flange to the second attachment flange, upon insertion of the separation member housing into the second portion of the central bore of the collar member, whereupon affixation of the second attachment flange to the first attachment flange affixes the separation member housing to the collar member.
The method further preferably comprises the step of providing at least a portion of an outer surface of the collar member with flattened sides, for facilitating clamping of the detensioning device in position over a strand chuck barrel and against an abutment.